The present invention is directed to a temperature stable multipole mass filter and method therefor and more specifically to a method for maintaining the R.sub.o parameter of a quadrupole mass spectrometer constant over a wide temperature range; in other words, to provide a mass to charge ratio (M/e) which does change with temperature so that if a single mass setting voltage is used rather than a scan the mass spectrometer will function effectively.
With the advent of mass spectrometers of the quadrupole type which are used for selecting a single mass peak (as opposed to the use of a mass scan) it is necessary to have accuracies as much as one part in 100,000. One critical parameter is the hyperbolic radius, also known as R.sub.o which is functionally related to the selected mass. This is obvious from examination of the standard Mathieu equations which are used to describe a quadrupole mass filter. In such a filter with a change in temperature both the rods and the rod mountings will expand. Normally such expansion would cause a change in R.sub.o and a concomitant change in the mass to charge ratio that is filtered by the device.
Attempts have been made to maintain the temperature constant to obviate this difficulty. However, during practical use of a multipole mass filter it is frequently expedient to maintain the filter at a temperature above ambient. This reduces the chance of condensation of gas molecules on the surface of a rod, thus reducing contamination which would distort the field patterns. But under these conditions if the temperature of the ambient environment changes, temperature change will occur in the mass filter itself to cause a thermal expansion or contraction.
A typical method of construction was described in an article by M. S. Story (one of the coinventors of the present application) at the Fourteenth National Vacuum Symposium AVS 1967 using molybdenum rods on aluminum oxide mounts. This provided a structure capable of constant resolution between 25.degree. C. and 400.degree. C. However, such a structure did not have a constant R.sub.o over this temperature range.
Thus, in summary there is a need for a device where, when a given mass to charge ratio (M/e) is to be filtered, R.sub.o is maintained constant throughout the length of the filter; this requires mechanical precision. Moreover, in order to maintain the filter stability over length of time, R.sub.o should stay constant regardless of environmental changes.